This invention relates to inkjet printers, and more particularly, to techniques for protecting print head circuitry.
An ink-jet printer is a type of non-impact printer which forms characters and other images by controllably spraying drops of ink from a print head. One conventional type of ink-jet print head consists of a replaceable cartridge or pen which is removably mounted to a movable carriage. The pen controllably ejects liquid ink through multiple nozzles in the form of drops that travel across a small air gap and land on a recording media.
Ink droplets are ejected from individual nozzles by localized heating. A small heating element, typically in the form of a thermal resistor, is disposed at each nozzle. An electrical current is passed through the element to heat it up. The heated element vaporizes a tiny volume of ink, which is ejected through the nozzle. The heating elements are commonly formed on a single silicon wafer chip, which make the replaceable pen easy to assemble and inexpensive to produce.
Current print head technology also implements firing logic on the print head. The firing logic is formed into the silicon wafer that forms the nozzles and heated firing elements. This reduces the number of connections to the pen and allows the print head to decode data at least partially on the fly. Logic-based pens are relatively inexpensive to produce as the logic circuitry is incorporated into the same silicon chip that is used to hold the heating resistors.
A problem encountered in such print heads concerns destructive overheating of the firing resistors (or other circuit components) as a result of power supply surges or interruptions. It would be desirable to protect these circuit elements from interruptions to avoid the destructive overheating. Any solution, however, must be relatively inexpensive because the pens are designed to be replaceable and/or disposable to satisfy other manufacturing goals of providing an efficient way to replenish spent ink supplies.
Accordingly, there is a need for a replaceable inkjet print head that protects the logic circuitry (namely, the firing resistors) against power supply interruptions, without increasing the manufacturing cost of replaceable pens.
This invention concerns an inkjet print head having power supply fault protection circuitry to guard against harmful and destructive effects on firing resistors resulting from power supply fluctuations. The power supply fault protection circuitry is integrated into a pen-based chip that also forms the firing elements and optionally the firing logic. As a result, the circuitry offers a low cost solution to problems associated with power supply fluctuations.
In a described implementation, the power supply fault protection circuitry has a power supply fault detector that detects if any one of a number of power sources are experiencing a fault condition (e.g., not present or not operating at proper levels). When a fault condition is detected, the fault detector outputs a xe2x80x9ckillxe2x80x9d signal. The protection circuitry also has a set of protection transistors, each coupled to the firing transistors of the inkjet print head. The xe2x80x9ckillxe2x80x9d signal turns on the protection transistors, which in turn turns off the firing transistors and overrides the firing signals. By halting firing, the protection circuitry protects the firing resistors from destructive overheating caused by fluctuations in the power sources.